Overview of setting up openvpn client using LuCI can be found here.
https://openwrt.org/docs/guide-user/services/vpn/openvpn/client-luci
See also the 'Alternative guide for OpenVPN client with LuCI' at the bottom of the above wiki page for detailed step by step instructions and tips. v1.1 pdf covers LEDE17/OpenWrt 18. v1.2 pdf for OpenWrt 19 using new .ovpn file upload function.